As a predictive candidate control method for improving conventional character string input efficiency, for example, there is a character string prediction method described in Patent Literature (PTL) 1. In the method, a plurality of characters are assigned to a character key according to a preset condition. A character string already inputted is combined with each character assigned to the character key, to generate a character string. For each generated character string, a word dictionary is searched for a character string whose pronunciation starts with the generated character string, as a result of which predictive character strings are generated from the corresponding character strings. Display order of the predictive character strings is determined by a predetermined procedure.